The present invention is directed to stretch wrap films and methods for their use. In particular, the present invention is directed to stretch wrap films having good physical properties in maximum stretch, holding force, puncture resistance, total energy dart drop, cling force and overall strength. The films of the present invention are particularly useful in hand wrapping applications.
The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the unitizing of palleted loads is a significant commercially important application of polymer film, including generically, polyethylene and other polyolefins.
Over-wrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In addition to machine wrapping, these products are utilized in hand wrapping operations. Pallets can be hand wrapped quickly, safely and economically. Hand wrapping is particularly useful in wrapping irregular or non-uniform pallets and articles. Hand wrap films exhibit toughness sufficient to hold the article. The industry also utilizes narrow width hand wrap films. These offer an alternative to adhesive based wrapping films, tape, twine, strapping or glue. The hand wrap films need to exhibit good cling force, but only to the film itself and not to the wrapped article. The hand wrap operations offer affordable, easy to use means of keeping loads clean, damage free and safe from pilferage. The hand wrap films should also exhibit good puncture and tear resistance.
Hand wrapping large pallets is finding acceptance in industry today. In this method, the film is arranged on a roll, however, it is hand held by the operator who walks around the goods to be wrapped, applying the film to the goods. The operator may install the roll of film so used on a hand-held wrapping tool for ease of use.
Some of the properties desired of a good stretch wrap film for hand wrapping are as follows: good cling or cohesion properties, puncture resistance, good machine direction tear resistance, good transparency, low haze, low stress relaxation with time, high resistance to transverse tear especially when under machine direction tension, producible in thin gauges, low specific gravity and thus high yield in area per pound, good tensile toughness, high machine direction ultimate tensile strength, high machine direction ultimate elongation, and low modulus of elasticity.
Currently, different grades of stretch wrap films are commonly marketed for different end uses according to overall film properties. For example, certain stretch wrap films have superior properties for load retention, but these films are characterized by having poor stretching characteristics. On the other hand, certain stretch wrap films having superior stretching properties have low load retention properties, thus limiting their use. Some of the broader categories include: general-purpose hand wrap films, heavy duty hand wrap films, and special formulation hand wrap films.
A need exists to develop general-purpose hand wrap stretch films. These films exhibit good properties at an acceptable cost that is usually lower than the other premium or specialty grades. The target for this grade is to maximize the film surface per amount of polymer used. The film still must exhibit the desirable physical properties outlined above. These films are particularly suitable for a wide range of hand wrap applications including general load unitization, use in long range load hauling, bar code scanning, and other shipping requirements. The products should provide good load retention characteristics and stretching characteristics while still maintaining other important stretch film properties.
Accordingly, the present invention provides for multi-layer films for hand wrap stretch film applications. The preferred resultant film has five layers with each layer being a single component or a blend of components to achieve the desired improved results.
In accordance with the present invention, attention has been focused on developing general-purpose hand wrap stretch films, while maintaining cost efficient production capabilities. Performance enhancements include improvement of load retention, tear resistance in machine direction (MD) and transverse direction (TD), puncture resistance, ultimate elongation, and overall strength. Desired properties of a xe2x80x9cgoodxe2x80x9d hand wrap stretch film are good cling or cohesion properties, good tear resistance in MD and TD directions, good clarity (low haze), high ultimate elongation, high tensile values, good stiffness (modulus), and high yield per pound.
The present invention is directed to co-extruded films comprised of at least 5-layers construction that are used for hand wrap film applications. In one embodiment, these multi-layer stretch wrap films are manufactured as cast films with conventional co-extrusion methods. Each of the five layers comprises a single polyolefin polymer or a mixture of polymers. When a mixture of polymers is utilized in a single layer, the polymers are introduced into an extruder to be mixed and extruded as a single layer. In one embodiment, the multi-layer film comprises a first outer layer, a first inner layer, a core layer, a second inner layer and a second outer layer. The first outer layer represents from 4 to 15 percent by weight of the film and comprises a blend of two different linear low-density polyethylene products. A first inner layer represents from 20 to 40 percent by weight of the film and comprises linear low-density polyethylene products. A core layer represents from 10 to 30 percent by weight of the film and comprises linear low density polyethylene products. A second inner layer represents from 20 to 40 percent by weight of the film and comprises linear low density polyethylene products. A second outer layer represents from 4 to 15 percent by weight of the film and comprises a blend of two different linear low density polyethylene products. The inner layers may comprise the same LLDPE composition as the core layer. The core layer is preferably extruded at a slower rate than the inner layers.
In another five layer film product of the present invention, each of the outer layers represents from 4 to 15 percent by weight of the film and each comprises a blend of 60 to 90 percent by weight of a first linear low density polyethylene product, and 10 to 40 percent by weight of a second different linear low density polyethylene product.
Another embodiment of the present invention provides a multi-layer film comprising two outer layers, two inner layers and a core layer. A first outer layer represents from 5 to 12 percent by weight of the film and comprises a blend of 70 to 80 percent by weight of linear low density-polyethylene/hexene copolymers, and from 20 to 30 percent by weight of linear low density-polyethylene/butene copolymers. A first inner layer represents from 30 to 35 percent by weight of the film and comprises linear low density-polyethylene butene/copolymers. A core layer represents from 15 to 20 percent by weight of the film and comprises a linear low density-polyethylene/butene copolymer products. A second inner layer represents from 30 to 35 percent by weight of the film and comprises linear low density-polyethylene/butene copolymer products. A second outer layer represents from 5 to 12 percent by weight of the film and comprises a blend of 70 to 80 percent by weight of linear low density-polyethylene/hexene copolymer products and 20 to 30 percent by weight of linear low density-polyethylene/butene copolymer products. Preferably, the linear low density-polyethylene/butene copolymer in each layer is the same polymer. Such a polymer has a melt index from 1.7 to 2.3 g/10 min. and a density of from 0.915 to 0.9198 grams per cubic centimeter.